1. Field of the Invention
The present invention relates to a polymer electrolyte membrane for a fuel cell, and a membrane-electrode assembly and a fuel cell system including the same. More particularly, the present invention relates to a polymer electrolyte membrane for inhibiting cross-over of a hydrocarbon fuel, and a membrane-electrode assembly and a fuel cell system including the same.
2. Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and a hydrogen fuel contained in a hydrocarbon-based material, such as methanol, ethanol, or natural gas.
Representative examples of fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell which uses methanol as a fuel.
A polymer electrolyte membrane fuel cell (PEMFC) has a high energy density, but requires a fuel reforming processor for reforming methane, methanol, natural gas, and the like, in order to produce a hydrogen-rich gas as the fuel gas.
By contrast, a direct oxidation fuel cell (DOFC) has an energy density that is lower than the polymer electrolyte membrane fuel cell, but does not need a fuel reforming processor.
A fuel cell includes a stack that actually generates the electricity. The stack includes several unit cells stacked in a multi-layer fashion. Each of the unit cells is made up of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly has an anode (referred to as a fuel electrode or an oxidation electrode) and a cathode (referred to as an air electrode or a reduction electrode) separated from each other by a polymer electrolyte membrane.
Perfluorosulfonic acid resin (e.g., NAFION®) can used as a material for forming the polymer electrolyte membrane. A polymer electrolyte membrane formed using perfluorosulfonic acid resin has an oxygen solubility, an electrochemical stability, and a durability that are higher than a hydrocarbon polymer membrane.
In general, a thicker perfluorosulfonic acid resin membrane at a thickness ranging from 50 to 175 μm provides better dimensional stability and mechanical properties than a thinner perfluorosulfonic acid resin membrane, but the thicker perfluorosulfonic acid resin membrane has a higher membrane resistance than the thinner perfluorosulfonic acid resin membrane. By contrast, the thinner membrane has a higher proton conductivity, but may also allow unreacted fuel gas and liquid to pass through its thinner polymer membrane resulting in a lost of unreacted fuel gas to thereby lower the performance of the fuel cell.